Human T cell leukemia virus type I (HTLV-I) is an RNA tumor virus identified as a causative agent of adult T cell leukemia and shown to be associated with several neurological and chronic disorders. An estimated 10 to 20 million people are infected by HTLV-I. Of these only a small percentage develop symptomatic disease, the majority of infections are silent. The mechanisms of the development and pathogenesis of HTLV-I associated diseases remain unknown. Studies of human viral isolates as well as evidence from animal models yield no data to correlate disease with genetic variation in the virus. A flow cytometric assay that measures binding of human T-lymphotropic virus type 1 (HTLV-1) virions to target cells was used to investigate the binding process and to screen for compounds affecting viral binding. The cell-free virus preparations used here mediated infection of rabbits and in vitro infection of certain target cell lines. Optimum binding was observed generally with T-cell lines while monocytic lines showed weak but positive viral attachment; exceptions to this pattern included certain transformed T-cell lines of rabbit or feline origin which showed poor binding to virus. Results using human T-cell targets showed that adenosine receptor type 2 antagonists effectively inhibit viral binding at concentrations below 10 micromolar. No inhibition was seen when antagonist was used to pretreat cells or was added post binding, suggesting direct interference with virus attachment. Efficient HTLV-1 binding requires divalent calcium ions and temperatures above 20 degrees C. Disruption of lipid rafts by cholesterol depletion compromised viral binding but cleavage of glycosyl-phosphatidylinositol linkages from target cell surface had no effect. Recent results show that treatment of HTLV-1 producing cell lines with tunicamycin results in the production of viral particles that are unable to bind to cells. Most recent studies focus on the use of inhibitors to treat rabbits infected with HTLV-1. A real time PCR assay has been adapted to give accurate measure of HTLV-1 proviral load with beta-globin as internal standard. Initial studies indicate that treatment with antagonists of the adenosine receptor type 2 at the time of infection with blood or cell-free virus lowers viral load.